It is often difficult or even impossible to defibrillate large patients or large animals with the energy delivered by commercial defibrillators. A defibrillator designed around two 18,000 joule capacitor banks has been constructed for the study of the defibrillatory energy requirements of large subjects. One of the banks can be wired to deliver up to 800, 1600, or 2400 volts: the other bank can be wired for 5,000, 10,000, or 15,000 volts. The capacitor banks are electronically switched to deliver rectangular, trapezoidal, truncated exponential or truncated exponential waveforms. The defibrillator is to be used in a systematic experimental study of the relationship between the various parameters of waveform and the effectiveness of the shock in reversing ventricular fibrillation in 90-110 kilogram calves. It is hoped that the data collected will be useful to those involved in the design of commercial defibrillators for use in large patients.